1. Field of the Invention
The present invention relates to a noise elimination device interposed in a coaxial cable in order to reduce noise entering a signal transmission line using the coaxial cable, as well as to a method for installing such a noise elimination device.
2. Description of the Related Art
In bi-directional CATV systems, in which television signals are sent from a central office to subscriber homes and signals are also transmitted from the subscriber homes to the central office, an upward signal frequency of 10 MHz to 55 MHz is often used for signals transmitted from the subscriber homes to the central office. However, the frequency of electromagnetic waves of noise generated by electronic household appliances lies in this frequency band as well, so that due to the electromagnetic waves generated by these electric household appliances, noise is incurred into the upward signal, and the problem of streamed noise affecting the central office side arises.
Therefore, ways to extinguish this noise have been studied, and noise generated in the center conductor of the coaxial cable was countered by providing a highpass filter, whereas noise generated in the outer conductor serving as the ground conductor was countered by providing a noise elimination unit as disclosed for example in JP H07-297035A. This noise elimination unit includes a coil made by winding a coaxial cable around a ring-shaped closed magnetic ferrite core, wherein the core conductor of that coaxial cable is connected in series with the core conductor of the coaxial cable to be installed, whereas its outer conductor is connected to the outer conductor of the coaxial cable to be installed, and the noise elimination unit is inserted between a relay amplifier and the cable.
However, to eliminate noise from both the center conductor and the outer conductor, a unit with a highpass filter and a noise elimination unit in which a coaxial cable is wound around a ferrite are both necessary, and their assembly is troublesome, and even providing both does not necessarily lead to a good noise elimination effect.
Furthermore, in CATV backbones, a large current for equipment operation is often transmitted over the coaxial cable, overlapping with the signal, and in that case, if the noise elimination unit disclosed in the above-mentioned publication is used in such a CATV backbone, then the coaxial cable wound around the ferrite core has to be a cable with large diameter, which can carry that current, which makes it difficult to wind around a ferrite ring, and even if it can be wound, the coil will be large, because the curvature of the coil cannot be made small, so that the unit itself becomes large and thus unsuitable for practice. Therefore, it cannot be installed easily in existing CATV facilities.
Moreover, to improve the noise elimination effect, the coil should have a large inductance with respect to the noise, and in the afore-mentioned upward signal frequency band, which is relatively low, it is necessary to wind the coaxial cable in many turns around the ferrite ring, so that also in this respect, the unit becomes large.
In view of these problems, it is an object of the first aspect of the present invention to provide a noise elimination device, which can be easily installed in a coaxial cable, and which can effectively eliminate noise from both the center conductor and the outer conductor, as well as a method for installing such a noise elimination device. It is an object of the fifth aspect of the present invention to provide a small noise elimination device that can be set up easily, even with a coaxial cable carrying a large current in superposition.